PLT-1909 Fixed error rendering pending posts (#3041)

* Fixed error caused by checking if a post is a system message

* Fixed trying to edit system posts when using the up arrow hotkey

webapp/components/post_info.jsx

@@ -33,7 +33,7 @@ export default class PostInfo extends React.Component {
         var post = this.props.post;
         var isOwner = this.props.currentUser.id === post.user_id;
         var isAdmin = TeamStore.isTeamAdminForCurrentTeam() || UserStore.isSystemAdminForCurrentUser();
-        const isSystemMessage = post.type.startsWith(Constants.SYSTEM_MESSAGE_PREFIX);
+        const isSystemMessage = post.type && post.type.startsWith(Constants.SYSTEM_MESSAGE_PREFIX);
 
         if (post.state === Constants.POST_FAILED || post.state === Constants.POST_LOADING || Utils.isPostEphemeral(post)) {
             return '';

webapp/components/rhs_comment.jsx

@@ -85,7 +85,7 @@ export default class RhsComment extends React.Component {
 
         const isOwner = this.props.currentUser.id === post.user_id;
         var isAdmin = TeamStore.isTeamAdminForCurrentTeam() || UserStore.isSystemAdminForCurrentUser();
-        const isSystemMessage = post.type.startsWith(Constants.SYSTEM_MESSAGE_PREFIX);
+        const isSystemMessage = post.type && post.type.startsWith(Constants.SYSTEM_MESSAGE_PREFIX);
 
         var dropdownContents = [];
 

webapp/components/rhs_root_post.jsx

@@ -41,7 +41,7 @@ export default class RhsRootPost extends React.Component {
         const user = this.props.user;
         var isOwner = this.props.currentUser.id === post.user_id;
         var isAdmin = TeamStore.isTeamAdminForCurrentTeam() || UserStore.isSystemAdminForCurrentUser();
-        const isSystemMessage = post.type.startsWith(Constants.SYSTEM_MESSAGE_PREFIX);
+        const isSystemMessage = post.type && post.type.startsWith(Constants.SYSTEM_MESSAGE_PREFIX);
         var timestamp = UserStore.getProfile(post.user_id).update_at;
         var channel = ChannelStore.get(post.channel_id);
 

webapp/stores/post_store.jsx

@@ -454,10 +454,11 @@ class PostStoreClass extends EventEmitter {
         for (let i = 0; i < len; i++) {
             const post = postList.posts[postList.order[i]];
 
-            // don't edit webhook posts or deleted posts
+            // don't edit webhook posts, deleted posts, or system messages
             if (post.user_id !== userId ||
                 (post.props && post.props.from_webhook) ||
-                post.state === Constants.POST_DELETED) {
+                post.state === Constants.POST_DELETED ||
+                (post.type && post.type.startsWith(Constants.SYSTEM_MESSAGE_PREFIX))) {
                 continue;
             }